As the popularity of smart phones and data oriented applications continues its rise, mobile network are facing difficulties in meeting fast growing traffic demand. In the near future it is expected that a significant proportion of population of the world's greatest cities will require mobile broadband services at megabit rates, and it has been conservatively calculated that this will require capacity densities of at least 1 Gbps/km2, and very probably 10 Gbps/km2. This is several orders of magnitude greater than that provided by existing network infrastructure, even if the most recent 4G proposals are employed. While some further spectrum may be available for this application, it is inevitable that the majority of required increase for capacity will have to be provided by reducing cell sizes, continuing the trend of the past two decades.
Since the introduction of the first IEEE802.11 standard in 1997, WLANs have become ubiquitous in homes, offices, airports and other public places as a way for wireless devices to access the Internet or a company Intranet. Although the IEEE802.11 group of standards is not the only standard applicable to WLANs, it is the pre-eminent standard and therefore, in the remainder of this specification, the term “WLAN” is to be understood as referring primarily to IEEE802.11, without excluding other standards defining wireless networks operating on similar lines.
There are essentially two types of WLAN topology: the ad-hoc network in which wireless devices communicate with each other without involving any central access points or any connection to a wired network, and, an infrastructure wireless network (of more relevance to the invention to be described) in which one or more access points (APs), connected to a (wired) backhaul network such as broadband Internet, provide a bridge to a number of wireless devices in wireless communication with the APs. The AP is thus the nearest equivalent to a base station in a wireless cellular telephone system. The coverage area provided by one AP is referred to as a “hotspot”. Although each hotspot has typically a size of only tens of metres, a larger area can be covered by using multiple geographically-overlapping APs. The hotspots may thus be regarded as cells and the WLAN may form a Small Cell Network (SCN). As in a cellular telephone system, handovers of mobile wireless devices from one AP to another are possible as the wireless devices move between APs. When a wireless device connects to a given AP it “associates” with that AP and starts a “session” for communication with that AP, thereby providing services to a user. Quality of Service, QoS, of those services provided by the operator in turn determines the Quality of Experience, QoE, perceived by the user.
Conventionally, therefore, the wireless device (also referred to as a terminal, client or UE) decides which AP to associate with, and this association is maintained for the duration of a session between the wireless device and AP, until the wireless link is terminated in some way, for example by the wireless device moving out of range of the AP. This causes a handover to another AP: the wireless device detects that a different AP would provide a better signal strength than the AP with which the wireless device is currently associated, so the wireless device “re-associates” with the new AP. The re-association process is basically the same as for association, except that if the new AP uses a different channel from the previous AP then the wireless device will need to retune its radio accordingly.
Small cell networks (SCN) cover a range of radio networks that can be generally classified into two categories based on whether or not the network is managed by operators. For example the low-power micro, pico and femto cells of a 3G or LTE-based wireless communication system are regarded as operator-managed SCN, and Wi-Fi networks (WLANs) can be regarded as non operator-managed SCN. These SCN are all based on the idea of deploying BSs much smaller than the traditional macro cell devices to offer extended coverage (to indoor area or coverage hole areas of the existing macro cells) or support high capacity demand in high-traffic areas.
Both the operator-managed SCN and Wi-Fi SCN have their own advantages. It is expected that, in the future, Wi-Fi SCN will remain a complementary tool to mobile networks but without replacing mobile networks. Although Wi-Fi will not have any performance guarantee, it plays a huge role in offloading network traffic because it has a low cost-per-bit by using license-exempt spectrum and sufficient spectrum to support high throughput rates to subscribers. In this way, WLANs may make a major contribution to a user's QoE.
The subscriber devices (UEs) may be of various types including laptop PCs, tablet computers, or mobile handsets. At least some of these client devices may be multi-RAT devices, capable of wireless communication in accordance with other radio access technologies (RATs) apart from Wi-Fi, in particular wireless cellular technologies such as 3G or LTE.
However, currently with the typical configuration of UEs that can support multimode RATs, they automatically select Wi-Fi access points (APs) for data traffic transmission once they are under the coverage of home or enterprise wireless local area. Moreover as already mentioned, UEs select which AP to associate with using only locally available information and most of them use signal strength as the dominant factor in selecting an AP. This can result in some RATs and APs becoming more congested than others, which will cause degradation in QoE. For example, when a lot of UEs congregate in a conference room, they all tend to select the same Wi-Fi AP even when multiple APs operating on different channels are available. Therefore, in order to ease the traffic congestion and improve the QoE in this kind of scenario, techniques are needed to efficiently manage the association of UEs with different APs using the same or different RATs, including different versions of IEEE802.11 as well as 3G and 4G RATs.